Octane demand has risen in recent years and the growth is likely-to continue in the United States. For example, it has been estimated that clear pool octane demand has been increasing by 0.15 units/year in recent years. The addition of alcohols and ethers such as methanol, ethanol and methyl t-butyl ether to gasoline to improve octane number and/or improve the effect of gasoline combustion in internal combustion engines on the environment has been the subject of a number of recent publications.
Methanol is generally made from synthesis gas and ethanol can be made by carbonylation of methanol or more usually from agricultural products by fermentation. Higher alcohols can also result from the catalyzed conversion of synthesis gas. Olefins such as ethylene and propylene are made in large quantities by the cracking of alkanes such as ethane, propane and naphtha. Potentially, additional large amounts of ethylene are available from natural gas by the oxidative coupling of the methane component.
Methanol, while effective if used essentially pure for transportation fuel, is not a good additive for gasoline and is also potentially available in large quantities by the partial oxidation the methane component in natural gas. Ethanol has shown promise as a gasoline additive, but i-butanol in particular is valuable as it can be dehydrated to i-butene and reacted with methanol to form methyl t-butyl ether which is an excellent octane improver that can be easily blended into gasoline. The i-butanol is also an effective octane improver. The methyl ether of i-pentanol is also an excellent octane improver for gasoline. U.K. Patent Application GB 2,123,411 describes a process for making a mixture of octane improving ethers by synthesizing an alcohol mixture containing methanol, ethanol, and higher alcohols and dehydrating the higher alcohols and etherification.
Because of the large amount of methanol available and its problems as a gasoline additive, processes which convert methanol to effective gasoline additives are valuable. Well-known is the Mobil process for converting methanol to gasoline-range hydrocarbons over an aluminum-containing molecular sieve.
In recent years there has been an upsurge in interest in the production of both chemicals and transportation fuels from non-petroleum carbon sources such as methane, tar sands, oil shale and the like. This interest has focused for lack of good direct conversion processes on indirect processes, which often go through a synthesis gas intermediate with subsequent conversion of the synthesis gas via Fischer-Tropsch and related processes to hydrocarbons and/or oxygenates. Oxygenates, particularly lower alcohols, are common products of such synthesis gas reactions, and high conversion, selective processes to convert an alcohol or a mixture of alcohols to higher molecular weight alcohols have substantial commercial potential.
Lower molecular weight oxygenated organic compound, in particular organic carbonates, are useful intermediates in the chemical field, and among these dimethyl carbonate is widely used as an additive for fuels, as an organic solvent and in the synthesis of other carbonates, both alkyl and aryl. Organic carbonates are, also, useful as synthetic lubricants, monomers for organic glass, plasticizers or as reagents in methylation and carbomethoxylation reactions for the preparation of phenol ethers, quaternary salts of ammonium, ureas, urethanes, isocvanates and polycarbonates.
Typical methods for preparation of alkyl carbonates consist in reaction of an alcohol with phosgene, as described for example in Kirk-Othmer, "Encyclopedia of Chemical Technology", 3rd ed., N.4, page 758. This procedure, however, has numerous technical problems (elimination of the hydrochloric acid produced in the reaction), as well as safety problems owing to the use of phosgene.
To overcome these technical problems, alternative methods of synthesis have been proposed, such as the oxidative carbonylation of methanol in the presence of catalysts based on palladium (see, for example, U.S. Pat. No. 4,361,519; German Pat. No.3,212,535 and British Pat. No.2,148,881), based on copper (see, for example, U.S. Pat. No. 3,846,468; U.S. Pat. No.4,218,391; and U.S. Pat. No.4,318,862) or based on cobalt (see, for example, Italian Patent Application No. 20809 A/90 and No.000374 A/91).
These methods of synthesis have, however, some disadvantages owing to the fact that reaction is carried out in a liquid phase and under basically homogeneous catalysis conditions. In the above methods, the reaction system has, in fact, a high sensitivity to the water produced which reduces both the selectivity of carbon monoxide toward formation of dimethyl carbonate, and the rate of reaction. There is difficulty in separating catalyst from reaction products and, when a catalyst based on copper is used, there is high corrosion of the reaction medium.
To overcome these disadvantages, processes carried out in a gas phase have been proposed wherein the organic carbonates are produced starting from methanol, carbon monoxide and dioxygen operating in the presence of an oxidative carbonylation catalyst. Examples of these catalysts are: supported salts and complexes of copper, systems which are generally rapidly deactivated and, in some cases, release hydrochloric acid and form corrosive mixtures (see, for example, U.S. Pat. No. 3,980,690, Italian Patent No. 1,092,951; U.S. Pat. No. 4,625,044; U.S. Pat. No. 5,004,827; and U.S. Pat. No. 4,900705); supported salts of palladium, systems which combined with nitrogen oxides, nitritoalkanes, dioxygen, carbon monoxide, produce organic carbonates but cause technical problems due to the use of nitritoalkanes and nitrogen oxides (see, for example, European Patent No. 425 197); and supported oxides, salts and complexes of cobalt (see, for example, European Patent Application No. 0 558 128 A2).
In the past various molecular sieve compositions natural and synthetic have been found to be useful for a number of hydrocarbon conversion reactions. Among these are alkylation, aromatization, dehydrogenation and isomerization. Among the sieves which have been used are Type A, X, Y and those of the MFI crystal structure, as shown in "Atlas of Zeolite Structure Types," Second Revised Edition 1987, published on behalf of the Structure Commission of the International Zeolite Associates and incorporated by reference herein. Representative of the last group are ZSM-5 and AMS borosilicate molecular sieves.
Prior art developments have resulted in the formation of many synthetic crystalline materials. Crystalline aluminosilicates are the most prevalent and, as described in the patent literature and in the published journals, are designated by letters or other convenient symbols. Exemplary of these materials are Zeolite A (Milton, in U.S. Pat. No. 2,882,243), Zeolite X(Milton, in U.S. Pat. No. 2,882,244), Zeolite Y (Breck, in U.S. Pat. No. 3,130,007), Zeolite ZSM-5 (Argauer, et al., in U.S. Pat. No. 3,702,886), Zeolite ZSM-II (Chu, in U.S. Pat. No. 3,709,979), Zeolite ZSM-12 (Rosinski, et al., in U.S. Pat. No. 3,832,449), and others.
Prior are liquid and gas-phase processes for synthesis of organic carbonates, in particular, dimethyl carbonate via Cu(II)-catalyzed oxidative carbonylation of methanol, offer limited reactor performance as the result of the effects of water formed as a co-product. Reactor water inhibits the catalytic reaction and limits reactant conversion to about 30 to 40 percent. In halide-containing fixed bed catalyst systems water leaches halide away from the catalyst resulting in long-term deactivation and excessive corrosion of metallic reactor and downstream hardware components (WO Patent No.87 07601).
There is, therefore, a present need for catalytic processes to prepare organic carbonates which do not have the above disadvantages. An improved process should, advantageously, be carried out in the vapor phase using a suitable catalyst system which provides improved conversion and yield. Such an improved process which converts lower value compounds to higher value organic carbonates would be particularly advantageous. Dimethyl ether is, for example, less expensive to produce than methanol on a methanol equivalent basis and its oxidative carbonylation to dimethyl carbonate does not produce water as a co-product.